When molten metal contacts atmospheric air, oxides and nitrides are formed which create fumes and can reduce the metallurgical purity of the metal. In the past a number of techniques have been used to shield the molten metal and thereby prevent the formation of oxides and nitrides. For example, covering powders have been spread on the surface of the molten metal to provide a barrier which isolates the molten metal from the atmosphere. This technique is clearly ineffective when the molten metal is poured in which case the problems of oxidation and fume generation remain.
More recently, carbon dioxide snow has been used to shield molten metal from atmospheric gases because it has beneficial effects over other types of industrial gases. In practice, carbon dioxide snow is produced by expanding liquid carbon dioxide through a nozzle to atmospheric pressure. The resulting snow is then injected through a discharge tube to the desired location.
It has been found that if the velocity of the carbon dioxide snow is too low, little or no snow reaches the surface of the molten metal because of a lack of jetting action. It has also been found that if the velocity of the carbon dioxide snow is too high, air is entrained within the snow. The presence of entrained air results in ineffective shielding. In addition, thermal exchange occurs between the hot air and the snow which results in untimely sublimation of the snow.
It has also been found that if the velocity of the carbon dioxide snow exiting the discharge tube is too high there results very poor snow production. This is because the carbon dioxide snow initially forms as very small pin point sized flakes. At very high velocities the pinpoint sized flakes do not increase in size and tend to sublimate before effective shielding occurs. At lower velocities the pin point sized flakes merge to give snow flakes of sufficient size to resist untimely sublimination.
As a result of the criticality of the velocity of carbon dioxide snow exiting the discharge tubes, the snow discharge tube is dimensioned to provide a desirable outlet velocity. More specifically, the velocity of the carbon dioxide snow flakes must be sufficient to provide the required kinetic energy for projecting or spraying the carbon dioxide snow to the desired location (e.g. the surface of the molten metal). In addition, it is necessary to avoid aspiration of air which can result in an untimely sublimation of a portion of the carbon dioxide snow which can adversely affect shielding efficiency.
Discharge tubes with a given internal diameter allow flow rates which are selected to provide the optimum velocity of the carbon dioxide snow passing therethrough for a given application. A given snow discharge tube therefore can operate only over a limited range of velocities. Thus, if different flow rates are required for different applications, it may be necessary to shut down the apparatus and change one or more of the discharge tubes. This procedure adds to the cost of operating the carbon dioxide snow discharge apparatus.